Cold-rolled steel sheet and method for manufacturing the same

ABSTRACT

A cold-rolled steel sheet has a partially recrystallized grain structure with a degree of unrecrystallization of 25% to 90% and a Rockwell hardness HRB of 83 or more, the cold-rolled steel sheet containing 0.01% to 0.15% C, 0.03% or less Si, 0.10% to 0.70% Mn, 0.025% or less P, 0.025% or less S, 0.01% to 0.05% Al, and 0.008% or less N on a mass basis, the remainder being Fe and unavoidable impurities, wherein the mean diameter of ferrite is 2 to 10 μm and these components satisfy Formula (1): (C %)+0.15×(Mn %)+0.85×(P %)≧0.21, wherein (M %) represents the content (mass percent) of an element M.

RELATED APPLICATIONS

This is a §371 of International Application No. PCT/JP2009/054102, with an international filing date of Feb. 26, 2009 (WO 2009/107856 A1, published Sep. 3, 2009), which is based on Japanese Patent Application Nos. 2008-050916, filed Feb. 29, 2008, and 2008-287692, filed Nov. 10, 2008, the subject matter of which is incorporated by reference.

TECHNICAL FIELD

This disclosure relates to a cold-rolled steel sheet which is suitable for use in clutch plates, synchronizer rings, and clutch discs that are mechanical parts of automotive transmissions and which has excellent strength, punching workability, and heat deformation resistance and also relates to a method for manufacturing the cold-rolled steel sheet.

BACKGROUND

Automotive transmissions are composed of clutch plates, synchronizer rings, clutch discs, and the like and have a function of transmitting a driving force and a function of absorbing heat generated by friction. Such parts are manufactured by punching steel sheets into ring-shaped plates. The transmissions have a structure, formed by piling up the ring-shaped plates, transmitting torque and need to have wear resistance and plate flatness. Therefore, properties required for steel sheets that are materials for the transmissions are excellent hardness, punching properties (punched surface properties such as flatness and burring), and deformation resistance during heating, that is, heat deformation resistance.

Among steels for machine structural use specified in JIS G 3311 are S35C cold-rolled steel sheets, which have been conventionally used principally as materials for clutch plates for automotive transmissions (hereinafter referred to as ATs). The S35C cold-rolled steel sheets are manufactured in such a manner that slabs are subjected to the following steps: hot rolling, pickling, annealing, and then cold rolling. Since hot-rolled steel sheets for the S35C cold-rolled steel sheets contain a large amount of C (about 0.35 mass percent) and therefore have high hardness, the hot-rolled steel sheets need to be annealed for a long time not less than several hours for the purpose of spheroidizing and softening a carbide before being cold-rolled. This is very disadvantageous in cost for automotive parts that need to be inexpensive.

JP 2003-277883 A discloses a technique in which the annealing of a hot-rolled steel sheet that is unsubjected to cold rolling is omitted. That is, the document proposes a cold-rolled steel sheet for AT clutch plates. The cold-rolled steel sheet has desired hardness, desired surface roughness, excellent wear resistance, and excellent punching workability and is manufactured in such a manner that a soft hot-rolled steel sheet containing 0.25 mass percent C is cold-rolled at a rolling reduction of 50% or more. The steel sheet has a problem that properties of a punched edge thereof are extremely deteriorated during punching because of residual stress caused by cold rolling and also has a problem that ring-shaped products have extremely low flatness due to thermal strain caused by an increase in temperature.

JP 2005-200712 A discloses a technique for improving residual stress caused by cold rolling. That is, this document proposes a cold-rolled steel sheet, having reduced residual stress, for AT clutch plates. This cold-rolled steel sheet is manufactured in such a manner that a steel sheet is cold-rolled and then further rolled at a rolling reduction of about 1% under light load using large rolls with a diameter of 300 mm or more. In this technique, the difference in strain between the front and back of the steel sheet is reduced and therefore properties of a punched edge thereof are improved during press punching; however, ring-shaped products have extremely low flatness because the residual stress in the steel sheet is not relieved and therefore deformation due to thermal strain caused by an increase in temperature cannot be avoided.

JP 2004-107722 A proposes a steel sheet, having excellent adhesion to a friction material, for AT clutch plates. The surface roughness thereof is optimized by a surface roughness-adjusting treatment such as pickling. The steel sheet is hot-rolled, pickled, annealed at 500-800° C. for three hours or more for the purpose of spheroidizing a carbide, temper- or cold-rolled at a rolling reduction of 1% or more, and then subjected to a surface roughness-adjusting treatment. Ring-shaped products have extremely low flatness due to residual stress caused by cold rolling. This has not been solved as similar to those described in JP 2003-277883 A and JP 2005-200712 A.

It could therefore be helpful to provide a cold-rolled steel sheet which is suitable for use in clutch plates, rings, and clutch discs and which has high hardness, excellent edge properties for punching, and excellent flatness at elevated temperatures and to provide an advantageous method for manufacturing the cold-rolled steel sheet.

SUMMARY

To eliminate negative effects caused by the residual stain of a conventional steel sheet for clutch plates, a cold-rolled steel sheet in which residual strain is relieved by annealing subsequent to cold rolling may be used as a material instead of directly using a cold-rolled steel sheet as a material in a conventional way. However, in the case of complete recrystallization by annealing, a required hardness cannot be obtained.

We discovered that a desired hardness is secured and problems due to residual strain, that is, the deterioration of edge properties for punching and the deterioration of flatness due to thermal strain caused by an increase in temperature are prevented in such a manner that the microstructure of steel is transformed into a partially recrystallized grain structure in which an unrecrystallized grain structure remains partly instead of transforming the steel microstructure into a completely recrystallized grain structure.

That is, we discovered: when a recrystallized grain structure and an unrecrystallized grain structure coexist, no rolling strain remains in the recrystallized grain structure and therefore the deterioration of edge properties for punching and the deterioration of flatness due to thermal strain caused by an increase in temperature are prevented. However, rolling strain remains in the unrecrystallized grain structure and therefore the residual strain and the refining of recrystallized grains combine to secure a required hardness.

We further discovered that C, Mn, and P, which are components of steel, significantly contribute to the hardness of a steel sheet and, therefore, the use of an appropriate amount of these components is effective in stably obtaining a required hardness.

We thus provide:

-   -   1. A cold-rolled steel sheet has a partially recrystallized         grain structure with a degree of unrecrystallization of 25% to         90% and a Rockwell hardness HRB of 83 or more and contains 0.01%         to 0.15% C, 0.03% or less Si, 0.10% to 0.70% Mn, 0.025% or less         P, 0.025% or less S, 0.01% to 0.05% Al, and 0.008% or less N on         a mass basis, the remainder being Fe and unavoidable impurities,         wherein the mean diameter of ferrite is 2 to 10 μm and these         components satisfy Formula (1):

(C %)+0.15×(Mn %)+0.85×(P %)≧0.21  (1)

-   -   where (M %) represents the content (mass percent) of an element         M.     -   2. A method for manufacturing a cold-rolled steel sheet includes         hot-rolling a slab at a finishing temperature not lower than the         Ar₃ transformation point, coiling at a coiling temperature of         580° C. to 750° C., pickling, cold-rolling at a rolling         reduction of 65% or more, and then annealing at a temperature of         680° C. or lower by continuously annealing, wherein the slab         contains 0.01% to 0.15% C, 0.03% or less Si, 0.10% to 0.70% Mn,         0.025% or less P, 0.025% or less S, 0.01% to 0.05% Al, and         0.008% or less N on a mass basis, the remainder being Fe and         unavoidable impurities, and these components satisfy Formula         (1):

(C %)+0.15×(Mn %)+0.85×(P %)≧0.21  (1)

-   -   where (M %) represents the content (mass percent) of an element         M.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a graph showing the relationship between the equation C*=(C %)+0.15×(Mn %)+0.85×(P %) and hardness (HRB).

DETAILED DESCRIPTION

Reasons for specifying the microstructure of a steel sheet as described above are described below.

Mean Diameter of Ferrite: 2 to 10 μm

The mean diameter of ferrite needs to be within an appropriate range to achieve a sufficient hardness. When the mean diameter of ferrite is greater than 10 μm, a desired hardness cannot be achieved. When the mean diameter of ferrite is less than 2 μm, the hardness is excessively high and therefore the press punching properties are low. The mean diameter of ferrite is preferably within a range from 4 to 8 μm.

The mean diameter of ferrite is determined by observing a cross section of a steel sheet that is perpendicular to the rolling direction thereof in accordance with the cutting method specified in JIS G 0551 (appendix).

Degree of Unrecrystallization: 25% to 90%

An important feature of a cold-rolled steel sheet is that the cold-rolled steel sheet has a partially recrystallized grain structure consisting of an unrecrystallized grain structure and a recrystallized grain structure. The steel sheet needs to contain a certain amount of unrecrystallized grains in which rolling strain applied to the steel sheet during cold rolling remains to secure the hardness of a steel sheet. That is, to balance high hardness, punching workability, and heat deformation resistance, the ratio of an unrecrystallized grain structure to a recrystallized grain structure is important.

The degree of unrecrystallization needs to be 25% or more to obtain a desired hardness by the effect of cold rolling. However, when the degree of unrecrystallization is greater than 90%, the number of recrystallized grains is extremely small. This causes a deterioration in punching workability and a deterioration in flatness due to an extreme increase in residual strain. Therefore, the degree of unrecrystallization is preferably 90% or less and more preferably within a range from 40% to 80%.

The degree of unrecrystallization may be determined in such a manner that the percentage (area percentage) of an unrecrystallized grain structure in a microstructure is determined by observing a cross section of a steel sheet that is perpendicular to the rolling direction thereof.

Reasons for specifying the composition of a steel sheet as described above are described below. The symbol “%” used for each component denotes mass percent unless otherwise specified.

C: 0.01% to 0.15%

C is an element that is important in view of the hardness and wear resistance of the cold-rolled steel sheet. An increase in content of C increases the hardness and wear resistance thereof. Therefore, to obtain a desired hardness and wear resistance, the content of C is 0.01% or more. However, when the content of C is greater than 0.15%, punching workability is deteriorated. Furthermore, the difference in deformation strain between the front and back is increased during punching and deformation due to thermal strain is increased during heating. This causes a deterioration in flatness of a punching material. Therefore, the content of C is limited to a range from 0.01% to 0.15%. The content of C is preferably within a range from 0.05% to 0.15% and more preferably 0.10% to 0.15%.

Si: 0.03% or less

When the content of S is greater than 0.03%, defects resulting from scales are likely to be caused on surfaces of a hot-rolled steel sheet and it is difficult to completely remove the scales by performing pickling after hot rolling. Therefore, the hot-rolled steel sheet is likely to have surface defects resulting from the scales and the surface condition of the steel sheet is deteriorated. This negatively affects surface properties of the steel sheet that has been cold-rolled and then annealed. Therefore, the content of S is limited to 0.03% or less. The content of S is preferably 0.02% or less and may be 0%. In current refining technology, the lower limit of the content of S is about 0.005% without significantly increasing the steel production cost.

Mn: 0.10% to 0.70%

Mn is an element that fixes S, which is present in steel as an impurity, in the form of a precipitate (MnS) to reduce negative effects caused by S. The content of Mn needs to be 0.10% or more to obtain this advantage. However, when the content of Mn is greater than 0.70%, the hardness of a steel sheet is excessively increased, thereby causing a deterioration in punching workability. This is because Mn hardens steel by solid solution hardening. When the content of Mn is greater than 0.70%, defects resulting from scales are likely to be caused on surfaces of a hot-rolled steel sheet and it is difficult to completely remove the scales by performing pickling after hot rolling. This negatively affects surface properties of the steel sheet that has been cold-rolled and then annealed. Hence, a desired surface roughness is no obtained. Therefore, the content of Mn is limited to a range from 0.10% to 0.70%. The content of Mn is preferably 0.50% or less and more preferably within a range from 0.20% to 0.50%.

P: 0.025% or Less

P is an element that hardens steel by solid solution hardening. However, when the content of P is greater than 0.025%, slab cracking and/or surface defects of steel sheets are caused. Furthermore, a significant increase in hardness of steel is caused and therefore punching workability is deteriorated. Therefore, the content of P is limited to 0.025% or less. The content of P is preferably 0.023% or less. When the content of P is less than 0.01%, the hardening effect thereof is low. Therefore, the content of P is preferably 0.01% or more.

S: 0.025% or Less

S is an element that is present in steel as an impurity. Coarse inclusions are formed when the content of S is greater than 0.025%. The inclusions cause work cracking, leading to a serious deterioration in punching workability. S affects the scale removal of hot-rolled steel sheets. When the content of S is greater than 0.025%, surface properties of a pickled steel sheet are deteriorated. This increases the surface roughness of the steel sheet that has been cold-rolled and then annealed. Therefore, the content of S is limited to 0.025% or less and is preferably 0.020% or less.

Al: 0.01% to 0.05%

Al is an element used for the deoxidization of steel. When the content of Al is less than 0.01%, a sufficient deoxidization effect cannot be obtained. When the content of Al is greater than 0.05%, such a deoxidization effect is saturated. Therefore, the content of Al is limited to a range from 0.01% to 0.05%. The content of Al is preferably within a range from 0.03% to 0.05%.

N: 0.008% or Less

N is an element that is present in steel as an impurity. When the content of N is greater than 0.008%, a steel sheet is excessively hardened and, therefore, punching workability thereof is deteriorated. Therefore, the content of N is limited to 0.008% or less and is preferably 0.005% or less.

The essential components are as described above. It is insufficient that each component satisfies the content range only. In particular, C, Mn, and P need to satisfy the following Formula (1):

C*=(C %)+0.15×(Mn %)+0.85×(P %)≧0.21  (1)

Components significantly affecting the hardness of a steel sheet are C, Mn, and P. C* is an index representing the hardness of the steel sheet. A reason for specifying C* is described below with reference to FIG. 1 prepared on the basis of Example 1 below. There is a proportional relationship between C* and hardness (HRB) as shown in FIG. 1. When the value of C* is 0.21 or more, the desired hardness is 83 or more in HRB. C* satisfies conditions defined by Formula (1).

Components other than those described above are Fe and unavoidable impurities. It is possible that the steel sheet contains other components than those described above unless the advantages obtained are reduced.

Reasons for specifying the hardness and surface roughness of the steel sheet as described above are described below.

Hardness (HRB): 83 or More

A transmission has a structure in which several ring-shaped plates prepared by punching a steel sheet are stacked to transmit a torque. Therefore, the steel sheet used needs to have wear resistance and therefore needs to have a hardness (HRB) of 83 or more, which is sufficient to secure the wear resistance thereof. When the hardness thereof is less than 83 in HRB, a reduction in wear resistance is problematic. Hence, the hardness thereof needs to be 83 or more in HRB. When the hardness thereof is greater than 95 in HRB, the shape of a punched piece is faulty or cracks or fractures are caused in the steel sheet during punching. Therefore, the hardness thereof is preferably 95 or less in HRB.

A reduction in surface roughness is preferable for an increase in wear resistance. Therefore, the steel sheet preferably has a surface roughness Ra of 0.3 μm or less on an arithmetic average basis. The lower limit of the surface roughness thereof is about 0.1 μm at the current technology level unless manufacturing cost is not significantly increased.

A method for manufacturing a cold-rolled steel sheet will now be described.

A slab having the above composition is hot-rolled at a finishing temperature not lower than the Ar₃ transformation point thereof. The hot-rolled steel sheet is coiled at a coiling temperature of 580° C. to 750° C., pickled, and then cold-rolled at a rolling reduction of 65% or more. The steel sheet is annealed at a temperature of 680° C. or lower in a continuous annealing furnace.

A method for producing the slab is not particularly limited and may be a usual one. In view of production efficiency and quality of the slab, a steel converter and a continuous caster are preferably used for such production and casting, respectively, of the slab.

In hot rolling, the finishing temperature needs to be not lower than the Ar₃ transformation point in view of the quality and rolling efficiency of the hot-rolled steel sheet. When the finishing temperature is lower than the Ar₃ transformation point, the ferritic transformation of the hot-rolled steel sheet is accelerated and therefore the following problem is caused: a problem that the hot-rolled steel sheet has a reduced hardness because coarse grains are formed in surface layers. In subsequent coiling, the coiling temperature needs to be within a range from 580° C. to 750° C. When the coiling temperature is lower than 580° C., crystal grains are extremely fine and the hot-rolled steel sheet is hardened by cooling strain, which inhibits cold rollability. When the coiling temperature is higher than 750° C., the mean diameter of ferrite is increased after coiling, surface properties of the steel sheet are deteriorated because the formation of scales on surfaces of the steel sheet is excessive accelerated, and the surface roughness thereof is significantly deteriorated. The coiling temperature is preferably within a range from 600° C. to 720° C. The Ac₃ transformation point can be determined by the measurement of thermal expansion using a differential thermal dilatometer or the like.

After the hot-rolled steel sheet is pickled by a usual process and scales are removed from surfaces of the hot-rolled steel sheet, the hot-rolled steel sheet is cold-rolled. The rolling reduction of cold rolling needs to be 65% or more. This is because the hardness is increased by causing ferrite grains to be fine and the surface roughness is reduced during annealing performed subsequently to cold rolling. The lower limit of the rolling reduction is not particularly limited. In the case of cold-rolling the hot-rolled steel sheet at a rolling reduction exceeding 85%, the following problems are concerned: the faulty shape of the hot-rolled steel sheet, a reduction in thickness accuracy of the hot-rolled steel sheet, and an extreme increase in load of a cold rolling mill. Therefore, the rolling reduction is preferably 85% or less.

The cold-rolled steel sheet is annealed in the continuous annealing furnace. It is particularly important that annealing is performed at a temperature not higher than the recrystallization finishing temperature after cold rolling. When the annealing temperature exceeds the recrystallization finishing temperature, substantially 100% of the microstructure is transformed into a recrystallized grain structure by annealing and therefore rolling strain induced during cold rolling is lost. Thus, a desired high hardness cannot be obtained. A partially recrystallized grain structure in which unrecrystallized grains and recrystallized grains coexist can be obtained by setting the annealing temperature to the recrystallization finishing temperature or lower.

The degree of unrecrystallization is determined by the annealing temperature. Hence, the degree of unrecrystallization can be adjusted to 25% or more by setting the annealing temperature to 680° C. or lower. The lower limit of the annealing temperature is not particularly limited and is preferably 500° C. or higher in view of the control of the temperature and atmosphere of the continuous annealing furnace and the production efficiency thereof. The cooling rate of the annealed steel sheet is not particularly limited and is preferably about 5 to 25° C./s. In view of the shape stability of the steel sheet and the adjustment of the surface roughness of the steel sheet during temper rolling in the case of performing temper rolling, it is advantageous that the steel sheet is heat-retained within a temperature range from 320° C. to 420° C. in the course of cooling.

The degree of unrecrystallization can be appropriately adjusted by controlling the annealing temperature. For the relationship between the degree of unrecrystallization and the annealing temperature, for example, the relationship between an apparent degree of unrecrystallization determined as described below and an annealing temperature for achieving the apparent degree of unrecrystallization is determined in advance and an annealing temperature for achieving a desired degree of unrecrystallization may be determined on the basis of this relationship:

(Apparent degree of unrecrystallization)=(HRB(P)−HRB(S))/(HRB(H)−HRB(S))×100(%)

wherein

-   -   HRB(P): the Rockwell hardness of a steel sheet annealed at a         predetermined temperature (B scale).     -   HRB(S): the Rockwell hardness of a steel sheet annealed at a         temperature at which a completely recrystallized grain structure         is obtained (B scale).     -   HRB(H): the Rockwell hardness of a steel sheet annealed at a         temperature at which recrystallization does not occur at all (B         scale).

For the purpose of eliminating a variation in hardness due to conditions of cooling subsequent to annealing, the apparent degree of unrecrystallization is preferably determined in such a manner that the cold-rolled steel sheet is annealed, water-quenched, and then measured for hardness.

The degree of unrecrystallization of a steel sheet can be estimated in such a manner that the relationship between the apparent degree of unrecrystallization determined as described above and the hardness of the steel sheet is determined in advance and the apparent degree of unrecrystallization is determined from the hardness of the annealed steel sheet on the basis of this relationship.

The annealed cold-rolled steel sheet may be temper-rolled under light load. This is because the surface roughness thereof is adjusted and the hardness thereof is further improved. To achieve a preferred hardness Ra of, for example, 0.3 μm or less, the rolling reduction is preferably 2% or more in terms of elongation. The upper limit of the rolling reduction is not particularly limited. When the rolling reduction is excessively high, the steel sheet is uneven in shape. In consideration of the capacity of a rolling mill used for temper rolling, the rolling reduction is preferably 5% or less in terms of elongation.

EXAMPLES Example 1

Slabs having the compositions shown in Table 1 were heated to 1200° C., hot-rolled at finishing temperatures not lower than the Ac₃ transformation points thereof, cooled on a run-out table, and then coiled at 600° C., whereby hot-rolled steel sheets with a thickness of 5 mm were obtained. After being descaled by pickling, the hot-rolled steel sheets were cold-rolled at a rolling reduction of 70%, whereby cold-rolled steel sheets with a thickness of 1.5 mm were obtained. After the cold-rolled steel sheets were degreased, the cold-rolled steel sheets were annealed at 650° C. in a continuous annealing furnace. The annealing time thereof was one minute. After being annealed, the steel sheets were cooled at a rate of 10° C./s, heat-retained at a temperature of 320° C. to 420° C. for 2.5 minutes, and then cooled to room temperature. The steel sheets were further rolled at a rolling reduction (elongation) of 3.0% under light load on a temper rolling line.

The following results are summarized in Table 1: results obtained by analyzing the steel sheets obtained as described above for mean diameter of ferrite, degree of unrecrystallization, surface roughness, hardness, punching workability, and heat deformation resistance.

Each of analysis items was measured by a corresponding one of methods below.

Mean Diameter of Ferrite

A sample (a cross section perpendicular to the rolling direction) was cut out of each steel sheet and then polished, whereby ferrite grain boundaries were brought into sight. After the sample was observed at 800 times magnification with an electronic microscope and then photographed, the mean diameter of ferrite was determined in accordance with a method of ferrite grain size test for steel (JIS G 0552 (1998)) by the cutting method specified in JIS G 0551 (appendix).

Degree of Unrecrystallization

The degree of unrecrystallization was determined in the same manner as that used to determine the mean diameter of ferrite. That is, a cross section of each steel sheet that was perpendicular to the rolling direction thereof was observed at 800 times magnification and the area percentage of an unrecrystallized grain structure was thereby determined. The area percentage thereof was determined to be the degree of unrecrystallization.

Surface Roughness

The arithmetic average surface roughness Ra was determined in accordance with the measurement method specified in JIS B 0601.

Hardness (HRB)

A sample with a size of 20 mm×60 mm was cut out of each steel sheet and then measured in accordance with the Rockwell hardness test method specified in JIS Z 2245. Ten portions of the sample were measured on the B-scale and the measurements thereof were averaged, whereby the hardness (HRB) was determined.

Punching Workability

A ring-shaped specimen having an inner diameter of 140 mm and an outer diameter of 160 mm was prepared from each steel sheet in such a manner that the steel sheet was punched with a press-type punching machine at a clearance equal to 10% of the thickness of the steel sheet (a thickness of 1.5 mm). The punched edge of the steel sheet that was perpendicular to the rolling direction thereof was observed with an optical microscope with a magnification of ten to 20 times. The steel sheet was evaluated in accordance with standards below:

-   -   Good (A): a punched edge having no crack, void, burr, or serious         shear droop.     -   Faulty (B): a punched edge having a crack, void, or burr.

Heat Deformation Resistance

The ring-shaped specimen used to evaluate punching workability was heated at 300° C. for 30 minutes and then air-cooled to room temperature. The camber of the resulting specimen was evaluated. A specimen with a camber of 0.1 mm or less is evaluated to be good.

In particular, the camber thereof was measured as described below. After both surfaces of the specimen heated and then air-cooled were polished with a piece of 800 or finer emery paper, the specimen was put on a platen. Circumferentially arranged ten portions of the specimen were measured for height with a contact-type height gauge and were also measured for thickness with a micrometer. The difference between the height and thickness of each portion was determined. The maximum difference was determined to be the camber.

TABLE 1 Mean Degree of diameter unrecrystal- Surface Hard- Steel components (mass percent) of ferrite lization roughness ness Punching Camber No. C Si Mn P S Al N C* (μm) (%) (Ra: μm) (HRB) workability (mm) Remarks a 0.140 0.007 0.45 0.020 0.004 0.035 0.005 0.225 4 75 0.20 84 A 0.08 Inventive example b 0.130 0.006 0.43 0.022 0.007 0.038 0.004 0.213 5 75 0.16 83 A 0.05 Inventive example c 0.150 0.005 0.47 0.021 0.008 0.041 0.005 0.238 3 70 0.20 88 A 0.07 Inventive example d 0.120 0.008 0.29 0.014 0.010 0.037 0.004 0.175 6 55 0.18 75 A 0.06 Comparative example e 0.110 0.007 0.41 0.024 0.006 0.040 0.005 0.192 7 60 0.24 79 A 0.07 Comparative example f 0.100 0.008 0.27 0.018 0.009 0.039 0.004 0.156 10 45 0.22 72 A 0.08 Comparative example g 0.115 0.007 0.50 0.025 0.008 0.038 0.005 0.211 8 50 0.20 83 A 0.06 Inventive example Note: Underlined values are outside an appropriate range.

As shown in Table 1, every inventive steel has a desired mean diameter of ferrite, degree of unrecrystallization, and surface roughness and is excellent in hardness (HRB), punching workability, and heat deformation resistance.

Example 2

Slabs having the compositions shown in Table 1 were heated to 1250° C., hot-rolled at finishing temperatures shown in Table 3, cooled on a run-out table, and then coiled at 650° C., whereby hot-rolled steel sheets with a thickness of 3 to 10 mm was obtained. After being descaled by pickling, the hot-rolled steel sheets were cold-rolled at a rolling reduction of 50% to 80%, whereby cold-rolled steel sheets with a thickness of 1.5 mm were obtained. After the cold-rolled steel sheets were degreased, the cold-rolled steel sheets were annealed at various temperatures not higher than 680° C. in a continuous annealing furnace. The annealing time thereof was one minute. After being annealed, the steel sheets were cooled at a rate of 10° C./s, heat-retained at a temperature of 320° C. to 420° C. for 2.5 minutes, and then cooled to room temperature. The steel sheets were further rolled at a rolling reduction (elongation) of 0% to 3.5% under light load on a temper rolling line.

The Ac₃ transformation points shown in Table 2 were determined in such a manner that a specimen was taken from each slab, heated at 1250° C. for 30 minute, cooled at a rate of 1° C./s, and then measured with a differential thermal dilatometer.

The following results are summarized in Table 3: results obtained by analyzing the steel sheets obtained as described above for mean diameter of ferrite, degree of unrecrystallization, surface roughness, hardness (HRB), punching workability, and heat deformation resistance.

TABLE 2 (mass percent) Ar3 transformation Steels C Si Mn P S Al N C* point (° C.) Remarks A 0.140 0.007 0.45 0.020 0.004 0.035 0.005 0.225 834 Inventive steel B 0.180 0.007 0.08 0.022 0.005 0.042 0.004 0.211 824 Comparative steel C 0.140 0.400 0.42 0.021 0.003 0.038 0.006 0.221 852 Comparative steel D 0.090 0.006 0.80 0.040 0.006 0.045 0.004 0.244 849 Comparative steel E 0.150 0.009 0.37 0.019 0.005 0.066 0.010 0.222 832 Comparative steel F 0.006 0.008 0.39 0.025 0.090 0.039 0.003 0.086 895 Comparative steel G 0.130 0.008 0.29 0.014 0.010 0.007 0.004 0.185 837 Comparative steel H 0.004 0.009 0.44 0.024 0.005 0.038 0.005 0.090 898 Comparative steel I 0.115 0.007 0.50 0.025 0.008 0.038 0.005 0.211 841 Inventive steel J 0.085 0.008 0.70 0.025 0.005 0.034 0.005 0.211 851 Inventive steel Note: Underlined values are outside an appropriate range.

TABLE 3 Hot Hot Degree rolling rolling Anneal- of finishing coiling Cold ing Temper Mean unre- temper- temper- rolling temper- rolling diameter crystal- Surface Hard- ature ature reduction ature reduction of ferrite lization roughness ness Punching Camber Steels No. (° C.) (° C.) (%) (° C.) (%) (μm) (%) (Ra: μm) (HRB) workability (mm) Remarks A 1 850 650 75 650 3.0 4 75 0.20 84 A 0.08 Inventive example 2 850 600 65 650 3.5 7 50 0.14 83 A 0.05 Inventive example 3 860 550 75 620 3.0 4 92 0.16 96 B 0.21 Comparative example 4 850 630 75 630 1.0 5 60 0.42 83 A 0.05 Inventive example 5 860 630 65 740 3.2 10   5 0.18 76 A 0.04 Comparative example 6 860 650 50 640 3.5 18  70 0.15 78 B 0.20 Comparative example 7 850 650 80 800 3.0 15  10 0.20 74 A 0.03 Comparative example 8 850 640 85 650 3.5 3 97 0.22 97 B 0.28 Comparative example 9 850 630 70 630 0 6 75 0.48 86 A 0.03 Inventive example B 10 850 650 65 630 3.2 8 85 0.33 88 B 0.34 Comparative example C 11 870 650 65 650 3.5 7 85 0.51 83 B 0.18 Comparative example D 12 870 650 80 640 3.5 5 80 0.44 90 B 0.22 Comparative example E 13 850 650 70 680 3.0 9 80 0.29 83 B 0.24 Comparative example F 14 890 650 75 650 3.0 24   0 0.16 57 B 0.10 Comparative example G 15 860 620 80 640 3.2 10  25 0.15 68 A 0.05 Comparative example H 16 890 600 75 650 3.0 16  15 0.20 59 A 0.07 Comparative example I 17 860 590 85 620 4.0 6 90 0.13 84 A 0.07 Inventive example J 18 860 600 70 650 3.0 9 83 0.14 83 A 0.06 Inventive example Note: Underlined values are outside an appropriate range.

As shown in Table 3, Steels A, I, and J, which are inventive steels, are useful in obtaining a desired mean diameter of ferrite, a desired degree of unrecrystallization, a desired surface roughness, excellent hardness (HRB), excellent punching workability, and excellent heat deformation resistance when finishing temperatures during hot rolling, rolling reductions during cold rolling, annealing temperatures, and rolling reductions (elongations) during temper rolling are within appropriate ranges.

In contrast, Steels B to H, which are comparative steels, have a mean diameter of ferrite, a degree of unrecrystallization, and a surface roughness of which one is outside an appropriate range although finishing temperatures during hot rolling, rolling reductions during cold rolling, and annealing temperatures are within appropriate ranges. Therefore, obtained hardness (HRB), punching workability, or heat deformation resistance is not as good as expected herein.

In Examples 1 and 2, the degree of unrecrystallization was determined as described below and results equivalent to those obtained from the apparent degree of unrecrystallization determined as described above were obtained:

(Apparent degree of unrecrystallization)=((HRB(P))′−(HRB(S))′)/((HRB(H))′−(HRB(S))′)×100(%)

wherein

-   -   (HRB(P))′: the Rockwell hardness of a steel sheet having a         partially recrystallized grain structure (B scale).     -   (HRB(S))′: the Rockwell hardness of a steel sheet having a         completely recrystallized grain structure (B scale).     -   (HRB(H))′: the Rockwell hardness of a steel sheet having no         recrystallized grain structure (B scale).

(HRB(S))′ and (HRB(H))′ were determined as described below. The unannealed cold-rolled steel sheets were heated at 580° C. or 780° C. for a time of 100 seconds or shorter and then water-quenched. Samples taken therefrom were measured for hardness (HRB) and observed for microstructure. After it was confirmed that the samples heated at 580° C. contained no recrystallized grain structure, these samples were measured for hardness (HRB), whereby (HRB(H))′ was determined. After it was confirmed that the samples heated at 780° C. contained a completely recrystallized grain structure, these samples were measured for hardness (HRB), whereby (HRB(S))′ was determined.

INDUSTRIAL APPLICABILITY

The following sheet can be obtained in such a manner that steel components are adjusted and the microstructure of steel is transformed into a partially recrystallized grain structure: a cold-rolled steel sheet which is suitable for use in automotive transmission parts such as clutch plates and which has high strength (high hardness), excellent punching workability, and excellent heat deformation resistance. Annealing subsequent to cold rolling may be performed for a short time shorter than one hour and can be performed using a continuous annealing furnace with extremely high production efficiency. Hence, the cold-rolled steel sheet can be manufactured without a significant increase in cost and therefore is suitable for use in automotive parts experiencing severe price competition. 

1. A cold-rolled steel sheet having a partially recrystallized grain structure with a degree of unrecrystallization of 25% to 90% and a Rockwell hardness HRB of 83 or more, the cold-rolled steel sheet containing: 0.01% to 0.15% C; 0.03% or less Si; 0.10% to 0.70% Mn; 0.025% or less P; 0.025% or less S; 0.01% to 0.05% Al; and 0.008% or less N on a mass basis, the remainder being Fe and unavoidable impurities, wherein the mean diameter of ferrite is 2 to 10 μm and these components satisfy Formula (I): (C %)+0.15×(Mn %)+0.85×(P %)≧0.21  (1) where (M %) represents the content (mass percent) of an element M.
 2. A method for manufacturing a cold-rolled steel sheet comprising: hot-rolling a slab at a finishing temperature not lower than the Ar₃ transformation point to form a hot-rolled sheet; coiling the hot-rolled sheet at a coiling temperature of 580° C. to 750° C.; pickling the hot-rolled sheet; cold-rolling the hot-rolled sheet at a rolling reduction of 65% or more to form a cold-rolled sheet; and annealing the cold-rolled sheet at a temperature of 680° C. or lower by continuously annealing, wherein the slab contains 0.01% to 0.15% C, 0.03% or less Si, 0.10% to 0.70% Mn, 0.025% or less P, 0.025% or less S, 0.01% to 0.05% Al, and 0.008% or less N on a mass basis, the remainder being Fe and unavoidable impurities, and these components satisfy Formula (1): (C %)+0.15×(Mn %)+0.85×(P %)≧0.21  (1) where (M %) represents the content (mass percent) of an element M. 